1. Field of the Invention
The present invention relates to cationic polymer systems, to a process for their preparation and to their use as components of binders for printing inks.
2. Description Of Related Art
The term printing ink as used in this description refers not only to printable materials having an actual color effect but also to diluted printing inks which do not impart color. This term further refers to printable, noncolored clearcoats, which are capable of being diluted with water.
In order to ameliorate emission problems that arise when printing inks are applied, organic solvents are increasingly being replaced by water. It generally is known that resins having acidic or basic groups are employed as the fundamental components of aqueous printing inks. These resins typically are completely or partially neutralized in order to render them soluble or dispersible in water. In the aqueous phase, the resins possess a high capacity for accommodating pigments and/or fillers, and can also be employed as polymeric emulsifiers in emulsion polymerization processes for the production of dispersions. Moreover, to optimize printing properties, printing inks contain, for example, pigments, latex particles, filler materials, waxes, antifoams, film-consolidation agents and, frequently, organic solvents as well.
In the preparation of pigmented printing inks, pigments usually are milled in a portion of the binder employed, or in special binders: the paste resins or grinding resins. Printing inks then are produced from the resulting ink bases or milling pastes by making up the latter with remaining binder to form an ink. The grinding resins employed for such ink bases should conform to a range of properties. One important property is good wetting capacity for the relevant pigments. In addition, the grinding resins should be highly compatible with the principal binder and should not, in the required amount, substantially alter the properties of the principal binder.
The grinding resins also should be of low viscosity in the aqueous phase. Pigmented ink bases having a pigment/binder ratio (PBR) of &gt;0.5:2, preferably at least 1:2, furthermore, should also be stable on storage so that the pigments do not settle or form agglomerates. The pastes also should not thicken, for example, by developing structural viscosity or thixotropy, since it is necessary for use in practice that they remain pumpable and flowable over a relatively long period of time. It also is advantageous to employ as grinding resins binders which are printable under the conventional conditions of printing processes and which also allow an acceptable pulp recovery quality on de-inking.
For these reasons, binders of cationic structure are preferably employed. For instance, DE-A 41 15 731 describes aqueous ink compositions in which the partial use of cationic resins is intended to produce the de-inkability preferred. The cationic resins described therein are commercially available acrylic resins.
Cationic polymer systems have been known for many years and are employed, in particular, for electrodeposition coating systems. Such resins are produced, for example, by reacting bisphenols with epichlorohydrin (1-step process leading to Taffy resins) or with bisepoxides (advancement reactions) followed by reaction with amines to give polymers having .beta.-hydroxyamine structures, which after neutralization with acids, carry cationic charges (Houben-Weyl, METHODEN DER ORGANISCHEN CHEMIE [Methods of Organic Chemistry], 4th Edition, Vol. E20, p.1891 ff., Georg Thieme Verlag, Stuttgart 1987). These resins, however, cannot be used as binders for printing inks because of the reactive epoxides they contain, which after secondary reactions with the paper fibers, prevent a separation of printing ink from the paper. In addition, because of the polymer size and the content of flexibilizing substructures, such resins exhibit glass transition temperatures which are too low for them to be employed as rapid-drying binders for blocking-free and tack-free printing inks.
Binders which are dilutable in water after protonation with acid are the subject of U.S. Pat. No. 4,871, 808. This document describes reaction products of epoxides, amines and butadiene/acrylonitrile copolymers which are employed as active crosslinking components for cathodic electrodeposition. However, because of their low glass transition temperature, they cannot be used for printing systems.
U.S. Pat. No. 5,324,404 also describes binders which are dilutable in water in the presence of acid. These systems, however, are unsuitable for use as the binder base of aqueous printing inks because the resins contain high proportions by mass of flexibilizing polyether functions.
U.S. Pat. No. 4,554,212 describes binders for cathodically depositable coating compositions which contain a cationic synthetic resin in aqueous phase. The binders described, however, are unsuitable for use in the printing sector because the cationic resins only yield tack-free coatings after baking with crosslinking components.
German Application P 43 14 298.2 discloses cationic printing inks whose synthesis involves the addition of epoxide-free binders based on resins having .beta.-hydroxyamine structures in the main chain. A grave disadvantage of the polymer resins described therein is their inadequate influence on water resistance and drying rate of the printing ink films, which are qualities for which improvement is sought. The disclosures of each of the documents discussed above are incorporated by reference herein in their entirety.